MOTOR TORQUE DEFINITIONS BASIC AND TUTORIALS

The torques described in the following
paragraphs are listed in the Standards. The minimum values are given
in Table 20-1.

∗The torque values with other than
rated voltage applied are approximately equal to the rated voltage
values multiplied by the ratio of the actual voltage to rated voltage
in the case of the pull-out torque, and multiplied by the square of
this ratio in the case of the locked-rotor and pull-in torque. †With
rated excitation current applied.

Locked-rotor torque is the minimum
torque, which the synchronous motor will develop at rest for all
angular positions of the rotor, with rated voltage at rated frequency
applied.

Pull-in torque is the maximum
constant-load torque under which the motor will pull into
synchronism, at rated voltage and frequency, when its rated field
current is applied. Whether the motor can pull the load into step
from the slip running on the damper windings depends on the
speed-torque character of the load and the total inertia of the
revolving parts.

A typical relationship between maximum
slip and percent of normal Wk2 for pulling into step is shown in Fig.
20-11. Table 20-1 specifies minimum values of pull-in torque with the
motor loaded with normal Wk2; these values are given
below. (See also Table 20-1.)

Nominal pull-in torque is the value at
95% of synchronous speed, with rated voltage at rated frequency
applied, when the motor is running on the damper windings.

Pull-out torque is the maximum
sustained torque which the motor will develop at synchronous speed
for 1 min, with rated voltage at rated frequency applied, and with
rated field current.

In addition, the pull-up torque is
defined as the minimum torque developed between standstill and the
pull-in point. This torque must exceed the load torque by a
sufficient margin to assure satisfactory acceleration of the load
during starting.

The reluctance torque is a component of
the total torque when the motor is operating synchronously. It
results from the saliency of the poles and is a manifestation of the
poles attempting to align themselves with the air-gap magnetic field.
It can account for up to 30% of the pull-out torque.

The synchronous torque is the total
steady-state torque available, with field excitation applied, to
drive the motor and the load at synchronous speed. The maximum value
as the motor is loaded is the pull-out torque, developed as a power
angle A = 90 DEG.